Hyper-V network adapters - 1 per VM?

Hi,

I have 2 VM's running on 1 host.  Do I need 2 virtual NIC adapters and two hardware ethernet cables?  Will using 1 VNIC and 1 hardware NIC for two VM's impact performance?  They're being used for Citrix?

Also what is the impact by having 2 servers using 1 physical network adapter and two vnics?

Thanks, Matt.